Temperature maintaining apparatus and temperature control apparatus and method therefor

ABSTRACT

A method and apparatus are provided for maintaining a temperature between a first value and a second value. The method and apparatus utilize a heat transfer device capable of cycling through a series of cycles, the device being capable of changing the temperature from the second value to the first value when operational. During each cycle the device transfers heat for a first period of time when the temperature reaches the second volume until the first value is reached and thereafter the device stops transferring heat. The first period of time is compared to a desired cycle-on period of time during each cycle and the second temperature value then is adjusted for a subsequent cycle of the device so the first period of time when the device transfers heat approaches the desired cycle-on period of time. There may also be provided temperature controls for a liquid heater having a jacket containing the liquid. The jacket has an inner heated wall and an outer wall spaced apart from the inner wall. The controls include a temperature sensor capable of fitting to the jacket so that the sensor contacts the inner wall and the liquid. A control module monitors temperature sensed by the temperature sensor. The control module causes the heater to stop heating when the temperature is heated by the heater to a first value and causes the heater to heat when the liquid cools to a second value lower than the first value. The control module senses overheating when temperature sensed by the sensor increases to a third value, higher than the first value, when the heater is not heating, and then causes the heater to shut down.

BACKGROUND OF THE INVENTION

[0001] This invention relates to temperature maintaining apparatusessuch as transit vehicle heaters and temperature control apparatuses andmethods therefor.

[0002] Various temperature maintaining apparatuses, particularlyheaters, operate in a cyclic fashion. When the temperature reaches acertain threshold, such apparatuses operate for a period of time untilthe temperature is brought to a certain level and thereafter theapparatus becomes nonoperational until the threshold is reached again.For example, in a heater the temperature may be the temperature of afluid and the heater operates when the temperature of the fluid drops toa set lower value. The heater continues to operate until the temperatureis raised to a higher level at which time the heater switches off untilthe temperature again drops to the lower level.

[0003] One specific type of heater is a transit vehicle heater whichoperates to heat the engine coolant. The temperature of the coolant issensed by at least one temperature sensor and the heater, typicallyoperating on diesel fuel, is fired until the temperature of the coolantis raised to a higher level. At this point the heater switches off.

[0004] This cycling on and off between preset lower and highertemperatures produces less than satisfactory performance under someconditions. People in a transit vehicle, for example, may perceive thatthe temperature of the air in the vehicle drops to an uncomfortablelevel when the heater cycles on and off between preset lower and uppertemperatures. This is because too much time may occur near the bottom ofthe cycle when the temperature is low and/or because the meantemperature is reduced below optimum levels. Standard on/off temperaturecontrols or thermostats are not capable of adjusting themselves to sucha situation.

[0005] Transit vehicle heaters, as well as some other types of heatersfor liquids, conventionally employ two different temperature sensors.One of the sensors contacts the liquid, typically a water/anti-freezemixture in the case of a transit vehicle heater, to determine thetemperature of the liquid in order to cycle the heater on and off asrequired to maintain the temperature of the liquid. A second sensorcontacts the inner wall of the jacket to sense an overheat condition.The need for two different sensors increases the cost and complexity ofthe units.

[0006] Transit vehicle heaters, and other types of heaters often must beinstalled in different configurations. For example, in the case oftransit vehicle heaters, the direction of flow of liquid through theheaters sometimes must be reversed according to the plumbingrequirements of a particular vehicle. Conventional transit vehicleheaters do not readily adapt to a reversal of liquid flow because oftemperature sensors placed adjacent the fluid inlet and fluid outlet.The heater controls are adversely affected when the fluid flow isreversed because of the effect this has on the readings of the sensors.

[0007] Also some conventional heaters require two sensors adjacent thefluid inlet and fluid output respectively and cannot operate with asingle sensor or with one of the sensors disabled. The controls have alack of flexibility in this respect.

[0008] It is an object of the invention to provide an improved heattransfer device, such as a transit vehicle heater, as well as controlstherefor and a method of maintaining a temperature, which are capable ofadapting to different temperature conditions to maintain a more constantmean temperature compared to a simple system where the device cycles onand off at fixed temperatures.

[0009] It is another object of the invention to provide an improvedheater for liquids, a method of controlling temperatures for liquids anda temperature control apparatus where a single sensor can act both tomeasure temperatures of the liquids to determine when the heater cycleson and off, and also acts as an overheat sensor for the heater.

[0010] It is a further object of the invention to provide an improvedheater for liquids, a method of controlling such heaters and controlsystems for such heaters which are adaptable to varying installationrequirements, such as reversing the flow of liquids through the heaters.

[0011] It is still further object of the invention to provide animproved heater for liquids, a method of controlling such heaters and acontrol systems for such heaters which can operate with two temperaturesensors, with one of the sensors removed or with one of the sensorsdisabled.

SUMMARY OF THE INVENTION

[0012] There is provided, according to a first aspect of the invention,a method of maintaining a temperature between a first temperature valueand a second, variable temperature value, the method utilizing a heattransfer device capable of cycling through a series of cycles. Thedevice is capable of changing the temperature from the second value tothe first value when operational. During each said cycle the devicetransfers heat for a first period of time when the temperature reachesthe second value until the first value is reached and thereafter thedevice ceases to transfer heat. The first period of time is compared toa desired cycle-on period of time during each said cycle and said secondvalue then is adjusted for a subsequent cycle of the device so saidfirst period of time when the device transfers heat approaches saiddesired cycle-on period of time.

[0013] There is provided, according to a second aspect of the invention,an apparatus for maintaining a temperature between a first temperaturevalue and a second temperature value. The apparatus includes a heattransfer device capable of cycling through a series of cycles. Thedevice is capable of changing the temperature from the second value tothe first value when operational. There are controls which control thedevice so during each said cycle the device transfers heat for a firstperiod of time when the temperature reaches the second temperature valueuntil the first value is reached and thereafter the device ceasestransferring heat. The controls compare the first period of time to adesired cycle-on period of time during each said cycle and said controlsadjust the second temperature value for a subsequent cycle of the deviceso said first period of time when the device transfers heat approachessaid desired cycle-on period of time.

[0014] There is provided, according to a third aspect of the invention,a heater for a liquid including controls for maintaining a temperatureof the liquid between a first value and a second lower value, the heaterbeing capable of cycling through a series of cycles. The controlscontrol the heater so during each said cycle the heater heats for afirst period of time when the liquid cools to the second temperaturevalue until the liquid is heated to the first value and thereafter theheater stops heating. The controls compare the first period of time to adesired cycle-on period of time during each said cycle and said controlsadjust the second temperature value for a subsequent cycle of the deviceso said first period of time when the heater heats approaches saiddesired cycle-on period of time.

[0015] There is provided, according to a fourth aspect of the invention,a control system for a heat transfer device, the control systemmaintaining a temperature between a first value and a variable secondvalue. The heat transfer device is capable of cycling through a seriesof cycles. The device is capable of changing the temperature from thesecond value to the first value. The control system controls the deviceso during each said cycle the device transfers heat for a first periodof time when the temperature reaches the second temperature value untilthe first value is reached and thereafter the device stops transferringheat. The controls compare the first period of time to a desiredcycle-on period of time during each said cycle and said control systemadjusts the second temperature value for a subsequent cycle of thedevice so said first period of time when the device transfers heatapproaches said desired cycle-on period of time.

[0016] There is provided, according to a fifth aspect of the invention,temperature controls for a liquid heater having a jacket containing theliquid. The jacket has an inner heated wall and an outer wall spacedapart from the inner wall. The controls include a temperature sensorcapable of fitting to the jacket so that the sensor contacts the innerwall and the liquid. A control module monitors temperature sensed by thetemperature sensor. The control module causes the heater to becomenonoperational when the temperature is heated by the heater to a firstvalue and causes the heater to operate when the liquid cools to a secondvalue lower than the first value. The control module senses overheatingwhen temperature sensed by the sensor increases to a third value, higherthan the first value, when the heater is nonoperational, and then causesthe heater to shut down.

[0017] There is provided, according to a sixth aspect of the invention,a heater for a liquid, the heater having a jacket containing the liquid.The jacket has an inner heated wall and an outer wall spaced apart fromthe inner wall. Temperature controls are operatively connected thereto,the controls including a temperature sensor capable of fitting to thejacket so that the sensor contacts the inner wall and the liquid, and acontrol module which monitors temperature sensed by the temperaturesensor. The control module causes the heater to stop heating when thetemperature is heated by the heater to a first temperature value andcauses the heater to heat when the liquid cools to a second temperaturevalue. The control module senses overheating when a temperature sensedby the sensor increases to a third temperature value, higher than thefirst temperature value, when the heater is not heating, and then causesthe heater to shut down.

[0018] There is provided, according to a seventh aspect of theinvention, a method of controlling temperature for a liquid heaterhaving a jacket containing the liquid. The jacket has an inner heatedwall and an outer wall spaced apart from the inner wall. The methodincludes fitting a temperature sensor to the jacket so that the sensorcontacts the inner wall and the liquid, monitoring temperature sensed bythe temperature sensor, causing the heater to stop heating when thetemperature is heated by the heater to a first temperature value andcausing the heater to heat when the liquid cools to a second temperaturevalue lower than the first value, detecting when temperature sensed bythe sensor increases to a third value, higher than the first value, whenthe heater is not heating, treating said third value as an overheatcondition and then causing the heater to shut down.

[0019] There is provided, according to an eighth aspect of theinvention, a temperature control apparatus for a liquid heater havingtwo fittings for connecting liquid conduits to the heater, one of thefittings being either an inlet or an outlet for liquid, another of thefittings being another of the inlet or the outlet for the liquid. Theheater has a first temperature sensor adjacent to said one fitting and asecond temperature sensor adjacent to said another fitting. Theapparatus includes a memory having values for a first temperature value,where the heater stops heating, a second temperature value where theheater commences heating, and a third temperature value, higher than thefirst value, where the heater is shut off. Both sensors are capable ofsensing the first temperature value and the second temperature value andone of the sensors also senses the third temperature value. Controlscompare temperature readings from each of the sensors with each of thefirst temperature value, the second temperature value and the thirdtemperature value. The heater has an overheat condition when atemperature exceeds the third temperature value, a high condition when atemperature exceeds the first temperature value, a normal condition whena temperature is between the second temperature value and the firsttemperature value and a low condition when a temperature is less thanthe second temperature value. The apparatus indicates the overheatcondition when one of the sensors indicates a temperature exceeding thethird temperature value, the high condition when either sensor indicatesa temperature exceeding the first temperature value, but neither sensorindicates a temperature exceeding the third temperature value, thenormal condition when either sensor indicates a temperature exceedingthe second temperature value, but neither sensor indicates a temperatureexceeding the third temperature value, and a low condition when bothsensors indicate temperatures less than the second temperature value.

[0020] There is provided, according to a ninth aspect of the invention,a method of controlling temperatures in a liquid heater having twofittings for connecting liquid conduits to the heater, one of thefittings being either an inlet or an outlet for liquid, another of thefittings being another of the inlet or the outlet for the liquid. Theheater has a first temperature sensor adjacent to said one fitting and asecond temperature sensor adjacent to said another fitting. The methodincludes retaining values for a first temperature, where the heaterstops heating, a second temperature where the heater commences heating,and a third temperature where the heater is shut off, both sensors beingcapable of sensing the first temperature value and the secondtemperature value and one of the sensors also sensing the thirdtemperature value, comparing temperature readings from each of thesensors with each of the second temperature value, the first temperaturevalue and the third temperature value. The heater has an overheatcondition when a temperature exceeds the third temperature value, a highcondition when a temperature exceeds the first temperature value, anormal condition when a temperature is between the second temperaturevalue and the first temperature value and a low condition when atemperature is less than the second temperature value. The overheatcondition is indicated when one of the sensors indicates a temperatureexceeding the third temperature value, the high condition when eithersensor indicates a temperature exceeding the first temperature value,but neither sensor indicates a temperature exceeding the overheattemperature, the normal condition when either sensor indicates atemperature exceeding the second temperature value, but neither sensorindicates a temperature exceeding the first temperature value, and a lowcondition when both sensors indicate temperatures less than the secondtemperature value.

[0021] There is provided, according to a tenth aspect of the invention,a temperature control apparatus for a liquid heater having a firsttemperature sensor and a second temperature sensor, the apparatusincluding a memory having values for a first temperature, where theheater stops heating, a second temperature where the heater commencesheating, and a third temperature where the heater is shut off, andcontrols which compare temperature readings from each of the sensorswith each of the first temperature value, the second temperature valueand the third temperature value. Both sensors are capable of sensing thefirst temperature value and the second temperature value and one of thesensors also senses the third temperature value. The heater has anoverheat condition when a temperature exceeds the third temperaturevalue, a high condition when a temperature exceeds the first temperaturevalue, a normal condition when a temperature is between the firsttemperature value and the second temperature value and a low conditionwhen a temperature is less than the second temperature value. Theapparatus indicates the overheat condition when one of the sensorsindicates a temperature exceeding the third temperature value, the highcondition when either sensor indicates a temperature exceeding the firsttemperature value, but neither sensor indicates a temperature exceedingthe third temperature value, the normal condition when either sensorindicates a temperature exceeding the second temperature value, butneither sensor indicates a temperature exceeding the first temperaturevalue, and a low condition when both sensors indicate temperatures lessthan the second temperature value.

[0022] There is provided, according to an eleventh aspect of theinvention, a method of controlling temperatures in a liquid heater, theheater having a first temperature sensor and a second temperaturesensor, the method including retaining values for a first temperaturevalue, where the heater stops heating, a second temperature value wherethe heater commences heating, and a third temperature value where theheater is shut off. Both sensors are capable of sensing the firsttemperature value and the second temperature value and one of thesensors also senses the third temperature value. Temperature readingsfrom each of the sensors are compared with each of the first temperaturevalue, the second temperature value and the third temperature value. Theheater has an overheat condition when a temperature exceeds the thirdtemperature value, a high condition when a temperature exceeds the firsttemperature value, a normal condition when a temperature is between thefirst temperature value and the second temperature value and a lowcondition when a temperature is less than the second temperature value.The overheat condition is indicated when one of the sensors indicates atemperature exceeding the third temperature value, the high conditionwhen either sensor indicates a temperature exceeding the firsttemperature value, but neither sensor indicates a temperature exceedingthe third temperature value, the normal condition when either sensorindicates a temperature exceeding the second temperature value, butneither sensor indicates a temperature exceeding the first temperaturevalue, and a low condition when both sensors indicate temperatures lessthan the second temperature value.

[0023] There is provided, according to a twelfth aspect of theinvention, a method of detecting a temperature sensor error in a heaterfor a liquid including a temperature sensor and controls for maintaininga temperature of the liquid between a first value and a second lowervalue, the heater being capable of cycling through a series of cycles.The controls control the heater so during each said cycle the heatercommences heating when the temperature sensor indicates that thetemperature has decreased to the lower value, the controls indicating atemperature sensor error if the temperature indicated by the sensor doesnot increase by a specified amount within a specified time after theheater commences heating.

[0024] The invention offers significant advantages compared to the priorart, particularly prior art heaters and other heat transfer deviceswhich control temperature by switching on and off at fixed temperaturevalues. By comparison the invention allow such a device to adapt tochanging ambient conditions. If temperatures fall, then a heatingdevice, for example, tends to operate longer until the temperaturereaches a set upper value. This lengthened period of time is sensed bythe invention and the temperature where the heater becomes operationalis raised. This typically causes the heater to cycle on and off at anincreased frequency. The effect is to maintain the mean temperature andgive an increased degree of comfort compared with a conventional system.

[0025] At the same the invention allows such heat transfer devices toadapt in other ways. For example, a prolonged period of cooling may besensed and the heater may be turned on even though the temperature hasnot fallen to a value which would normally trigger operation of theheater. This again maintains a minimum mean temperature and increasesthe degree of comfort.

[0026] The invention also permits a single temperature sensor to replacetwo different temperature sensors normally used for temperatureregulation and for sensing an overheat condition. This reduces thenumber of parts required for devices such as transit heaters andsimplifies assembly.

[0027] The invention offers significant advances and increasesadaptability of various heat transfer devices such as heaters and hotliquid transit heaters in particular. For example, where such a heaterhas two different heat sensors the system can operate withoutinformation on which sensor is which or in which direction liquid flowsthrough the heater. Moreover, if one sensor is removed, or one becomesfaulted, then the system can adapt and still operate correctly.

BRIEF DESCRIPTION OF THE DRAWINGS

[0028] In drawings which illustrate embodiment so the invention:

[0029]FIG. 1 is an exploded isometric view of a transit heater accordingto an embodiment of the invention;

[0030]FIG. 2 is an enlarged front, side isometric view of the burnerhead assembly thereof;

[0031]FIG. 3 is an exploded, isometric view thereof;

[0032]FIG. 3a is a phantom view of the control module assembly of FIG. 3shown in an alternative position rotated 18020 from the position of FIG.3;

[0033]FIG. 4 is a fragmentary, sectional view showing a fragment of thejacket of the heater and a temperature sensor mounted thereon;

[0034]FIG. 5 is a flowchart of the first part of a method according toan embodiment of the invention;

[0035]FIG. 6 is a continuation of the flowchart of FIG. 5;

[0036]FIG. 7 is a continuation of the flowchart of FIG. 6;

[0037]FIG. 8 is a continuation of the flowchart of FIG. 7;

[0038]FIG. 9 is a chart showing temperature conditions and temperaturedesignations for the heater of FIG. 1;

[0039]FIG. 10 is a graph showing how temperature is regulated for aconventional heater as well as showing an overheat condition;

[0040]FIG. 11 is a graph showing temperature changes during operation ofthe heater of FIG. 1; and

[0041]FIG. 12 is a graph similar to FIG. 10 showing operation of theheater during a lingering drop off.

[0042]FIG. 13 is a fragmentary plan view of the burner head housing,partly broken away to show two air filter mounts and an air filtercorrectly mounted on one of the mounts;

[0043]FIG. 14 is a fragmentary view of a portion of FIG. 13 showing anair filter incorrectly mounted on one of the mounts;

[0044]FIG. 15 is a fragmentary sectional view of the burner headassembly showing the air compressor, air filter, air filter mount andair conduits extending between the filter and the compressor;

[0045]FIG. 16 is a fragmentary sectional view of the housing showing arod preventing an air filter from fitting an incorrect air filter mount;

[0046]FIG. 17 is a fragmentary sectional view showing a portion of thehousing located adjacent to the control module and air filter, in arotationally nonaligned position;

[0047]FIG. 18 is a fragmentary view thereof showing the portion of thehousing moving towards the control module in a rotationally alignedposition;

[0048]FIG. 19 is a view similar to FIG. 18 showing the portion of thehousing fitted onto the control module and against the air filter;

[0049]FIG. 20 is a rear, elevational view of the burner head assemblyshowing the burner, fuel pump, compressor and associated transformer;

[0050]FIG. 21 is a fragmentary sectional view showing two portions ofthe housing with the control module fitted therebetween and the circuitboard of the module extending outwardly to connect with two electricalsockets;

[0051]FIG. 22 is a fragmentary sectional view showing the Hall effectsensor and magnet on the fan blade assembly for the heater of FIG. 1;

[0052]FIG. 23 is a diagram showing the closed loop control system forthe motor thereof;

[0053] FIGS. 24-27 together comprise a flowchart of the system fordistinguishing overcurrent faults from voltage changes occurring whenthe engine of the vehicle starts;

[0054]FIGS. 28 and 29 together comprise a flowchart of the system forselectively soft starting the coolant pump;

[0055]FIG. 30 is a graph representing voltage changes when the enginestarts;

[0056]FIG. 31 is a diagrammatic representation of the heater and coolantpump of the vehicle; and

[0057]FIG. 32 is a flowchart of the system for monitoring sputteringflames and determining whether or not the heater should be shut off.

DETAILED DESCRIPTIONS OF THE PREFERRED EMBODIMENTS

[0058] Referring to the drawings, and first to FIG. 1, this shows atransit heater 30 according to an embodiment of the invention. Theheater include a heat exchanger 32 equipped with a primary temperaturesensor 34 and a secondary temperature sensor 36. The latter is optionalas described below. There are fittings 40 and 42 which serve as inletsand outlets for liquid, a mixture of water and anti-freeze being theusual liquid for a transit vehicle heater. Mounting brackets 44 and 46are connected to the heat exchanger for mounting the heater on a transitvehicle or similar application. A combustion tube 48 fits within theheat exchanger and a burner head assembly 50 fits over the combustiontube. The assembly is secured to the heat exchanger by bolts 52 and 54.

[0059] The burner head assembly is shown in better detail in FIGS. 2 and3. The assembly includes a burner head assembly housing 51, cylindricalin this example, having two portions, a blower housing 56 and a headflange assembly housing 66. The blower housing 56 has a splash guard 58connected thereto. There is an air intake behind the splash guard. Ablower 60 is mounted on shaft 62 of electrical motor 64. Burner headflange assembly housing 66 is at the end of the burner head assemblyopposite blower housing 56. The motor is mounted on the inside of theflange assembly housing 66 by means of bolts 68. A gear 70 is mounted onthe shaft of the motor and engages another gear 72 which drives acompressor 130 and a fuel pump 131 on the opposite side of the headflange assembly housing as seen in FIG. 20. There is a fitting 74 forconnecting a fuel line to the fuel pump.

[0060] Referring again to FIG. 20, there is a burner 132 having a nozzle134 provided with ignitors 136 and 138. The ignitors are connected totransformer 140 which receives electrical current through cable 142connected to internal electrical connector 144. Cable 146 and internalelectrical connector 148 are provided for the fuel solenoid valve.

[0061] Control module assembly 80, shown in FIG. 3, fits between the twoportions of the burner head assembly housing 51, namely head flangeassembly housing 66 and blower housing 56. The assembly includes anannular member, or module housing 82, having a central aperture 84 whichfits over the motor. The module housing 82 has a circular outer surface83 which is adjacent to the exterior of the burner head assemblyhousing. As seen in FIG. 21, the housing 56 has an annular recess 160provided with a shoulder 162 while housing 66 has a corresponding recess164 and shoulder 166. The control module has shoulders 168 and 170 whichbutt against the shoulders 162 and 166 to provide a sealing fit betweenthe control module and the burner head assembly housing.

[0062] There is a first cylindrical socket or mount 86 for receiving airfilter 88 as seen in FIG. 3. Likewise there is a second cylindricalsocket or mount 87 for receiving the air filter when the control moduleis rotated 180° as shown in FIG. 3a. A circuit board 150 is locatedwithin housing 82 as shown in FIG. 21. As shown in FIG. 3, a temperaturesensor plug 92 is received within a socket 98. There is also anauxiliary plug 96 and a socket 94 for receiving a data link plug.

[0063] Referring to FIGS. 3 and 3a, the control module assembly 80 canbe rotated 180° to an alternate position. This is done so that theexternal electrical connectors, such as connector 172, can be rotated toone side or the other of the housing 51 according to the requirements ofthe vehicle where the heater is being installed.

[0064] As shown in FIG. 15, housing 66 has an air conduit or passageway180 which communicates with air compressor 130. Likewise control modulehousing 82 has an air passageway 182 which aligns with air conduit orpassageway 180 and communicates with the air filter mount 86 for theposition of the control module housing 82 shown in FIG. 3. Thepassageways 182 and 180 therefore allow air to pass from the air filterto the air compressor. The air filter mount 87 also has an airpassageway 184, shown in FIG. 16, which is not used for air in thepositions shown in FIG. 3 and FIG. 16. This is because the aircompressor is on the opposite side of the assembly. It is desirable toprevent maintenance personnel from inadvertently mounting air filter 88in the wrong air filter mount, which would be mount 87 in FIGS. 3 and16. For this purpose a pin 186 is connected to the housing 66 in aposition such as to extend through the air passageway 184 as shown inFIG. 16. If an attempt is made to fit air filter 88 into the wrong mount87, then pin 186 prevents this from happening since the filter contactsthe pin and cannot be inserted properly into the mount as shown in FIG.16.

[0065]FIGS. 13 and 14 illustrate how blower cover 56 holds the airfilter properly in position. There is a member or pin 190 inside theblower housing 56 which extends towards the air filter when the airfilter is properly positioned as shown. The pin has an outer end 192which fits within flange 194 of the filter and secures the filter inposition. A flange 156 extends between the pin and the rest of thehousing apart from the outer end of the pin. The pin 190 is properlydimensioned so that the outer end 192 just contacts the filter when thefilter is properly positioned.

[0066] There is a similar pin 196 inside the housing on the sideopposite pin 190. It may be observed however that pin 196 is longer thanpin 190. As shown in FIG. 14, the outer end 198 of pin 196 contacts thefilter 88, if the filter is improperly positioned in mount 87, beforethe housing 56 is properly seated on control module 82. The pin 196accordingly prevents the blower housing from fitting properly on thecontrol module housing if the filter is improperly positioned.

[0067] FIGS. 17-19 illustrate a mechanism for preventing the pin 190from dislodging the filter when the blower housing 56 is being mountedon the control module housing 82. There is a key 200 on the blowerhousing 56 and a slot 202 on the module housing 82. There are three morespaced apart similar keys and similar slots on the housings. The keysfit within the slots when the housings 56 and 82 are properly alignedand allow the housings to fit together as shown in FIG. 19. If howeverthe housings are not rotationally aligned, as shown in FIG. 17, then thekeys 200 contact housing 82 and prevent the housings from fittingtogether.

[0068] Referring to FIG. 21, the control module housing 82 has aninternal circuit board 150. The control module housing as well as thecircuit board extend from interior 152 of the burner head assembly toexterior 154 thereof. The circuit board is connected to a pair ofexternal, electrical connectors 156 and 158, thereby allowing externalwiring harnesses to be connected to the control module without requiringwiring harnesses to pass through the housing of the burner assembly.Internally the circuit board is connected to such internal electricalconnections as auxiliary plug 96 and socket 98.

[0069] Details of the temperature sensor 34,and a fragment of the heatexchanger are shown in FIG. 4. The sensor includes a body 100 with ahexagonal outer portion 102 and a male threaded, hollow inner portion104. The male threaded portion is received in a female threaded socket106 in outer wall 108 of jacket 110 of the heater which surrounds theheat exchanger. The jacket has an inner wall 112 which is exposed toflame 114. Liquid 116, typically water and anti-freeze, is locatedbetween the inner wall and the outer wall. The sensor has a springloaded shank 118 which is biased against the inner wall 112. The sensorhas electrical contacts 120 and 122.

[0070] In a conventional heater a sensor, such as sensor 34, wouldfunction as either a temperature sensor for cycling the heater on andoff or as an overheat sensor. With reference to FIG. 10, a normaltemperature sensor reads coolant temperatures and turns the burner onwhen the temperature reaches a lower level, 65° C. in this example. Sucha temperature sensor has a portion immersed in the coolant. The burnerthen operates until the temperature reaches a first value, an upperlimit of 85° C. in this example, as illustrated at 124 in FIG. 10. Atthis point the controls turn the burner off and the temperaturegradually falls to a second value, 65° C. again at 126 and the burner isoperated again.

[0071] However, heaters conventionally also have an overheat sensorwhich senses, for example, the absence of coolant. If there is nocoolant in the jacket, then the inner wall 112 overheats and the heateris shut down. An overheat sensor is therefore in contact with the innerwall of the jacket. Sensor 34, however, serves both functions, to sensetemperature of the coolant, as well as sensing overheating of theheater. This is done utilizing the structure of the sensor shown as wellas appropriate programming of the control module.

[0072] With reference to FIG. 10, the heater is cycled off at points 124and 128 when the maximum temperature is reached. Under overheatconditions, however, the temperature sensed by the sensor continues toincrease after the burner cycles off as indicated at 130 in FIG. 10.Accordingly, the control module senses an overheat condition and shutsdown the heater when the sensor 34 indicates a third value, 90° C. inthis example, which exceeds the normal maximum temperature, after theheater has been cycled off. Thus a single temperature sensor can fulfillboth functions described above.

[0073] The heater is also adaptable to changing ambient conditions. Anormal on/off temperature control, as shown in FIG. 10, may lead toproblems when temperatures drop. Referring to FIG. 11, the system mayaim at providing a mean temperature of 75° C. by cycling on the heaterwhen the coolant reaches a lower temperature of 65° C. The heater thencycles on until the higher temperature of 85° C. (also referred to asthe first temperature value herein) is reached, at which time the heatercycles off. However, at lower ambient temperatures it takes longer forthe heater to heat the coolant to the higher temperature. For example,the time increases from T1 to T2 as a result of cooling conditions. Theeffect of this is to lower the mean temperature below 75° C. This maymean that the temperature inside a bus, for example, may be too cool forthe occupants.

[0074] The invention overcomes this problem by monitoring the time T1 orT2 which it takes for the heater to heat the coolant to the highertemperature. When this time increases, the control module increases thetemperature where the heater cycles on (also referred to as the secondtemperature value herein). For example, in FIG. 1 the cycle-ontemperature is increased to 68° C. since the controls determined that T2exceeds T1. The cycle-on temperature is also lowered if the timeinterval to heat the coolant shortens. The cycle-on temperature isvaried in this example between specific limits, the lower limit being65° C. and the upper limit being 78° C. which is a fixed amount, ΔT,below the maximum temperature. The programming of the controls is setout in detail in FIGS. 5-8.

[0075] The algorithm can only modify the temperature within the standardrange of 65° C. to 85° C. The cycle-off temperature is never adjusted.This ensures that in the worst-case scenario the heater would justrevert back to the standard control method. The algorithm updates thecycle-on temperature once every cooling curve. This ensures that theheater will rapidly adapt to any changes in the parameters of theheating system. By using the heat time to calculate the new cycle-ontemperature, every parameter of the heating system is taken intoconsideration.

[0076] Details of the algorithm follow:

[0077] Adjusting Cycle-On Temperature

[0078] a) The current cycle-on temperature threshold is adjusted at theend of each heating interval (i.e. on entry to purge state)

[0079] i) based on the formula:

[0080] New-current cycle-on temperature threshold=

[0081] (1−(target heat time/actual heat time))* maximum temperaturechange+current cycle-on temperature threshold

[0082] (1) target heat time defines the ideal flame-on-time

[0083] (2) actual heat time is measured as duration of heating cyclestarting with entry into ignition state and ending with cycle-off event,but qualified by at least one temperature sensor reading above currentcycle-on temperature threshold

[0084] (3) maximum temperature changes intended to limit the amount ofadjustment made in one heating cycle

[0085] ii) current cycle-on temperature threshold is not permitted toexceed (cycle-off temperature threshold- minimum delta-T), or be lessthan cycle-on temperature threshold.

[0086] Timed Cycle-On

[0087] a) a cycle-on event is forced if cooling time since the end ofthe previous heating cycle exceeds the current maximum cool time and atleast one temperature sensor is reading less than cycle-off temperaturethreshold- minimum delta-T

[0088] i) a current maximum cool time timeout can occur in standby orstandby supplemental states

[0089] ii) a current maximum cool time timeout causes the currentcycle-on temperature threshold to be updated with the greater of the twotemperature sensor readings.

[0090] b) for each consecutive heating cycle begun due to currentmaximum cool time, the value of current maximum cool time is doubled(for use in the next cycle)

[0091] i) in the subsequent heating cycle, the value of current cycle-ontemperature threshold is adjusted per the formula above, as usual.

[0092] ii) if, in the current heating cycle, actual heat time exceedstarget heat time, then current maximum cool time reverts to defaultmaximum cool time.

[0093] Short Cycle

[0094] a) While purge or purge error state, if T1 and T2<currentcycle-on temperature threshold, the system does not wait for completionof purge state, but immediately cycles-on the heater, i.e. abandonspurge state, and skips standby state. If the heater is operating insupplemental mode, it also skips the pre-run state.

[0095] Initial Conditions

[0096] a) at power-up, the value of current cycle-on temperaturethreshold reverts to the value of cycle-on temperature threshold, andthe value of current maximum cool time reverts to the value of defaultmaximum cool time. Also, the cooling time timer is reset.

[0097] b) At switch-off, the value of current maximum cool time revertsto the value of default maximum cool time. Also the cooling time timeris reset.

[0098] As described above, the heater can operate with a singletemperature sensor 34 as shown in FIG. 1. Optionally however a secondsensor 36 may be used for some applications. Typically one sensor isadjacent the hot end of the heater, sensor 34 in this example, andanother sensor is adjacent the cool end of the heater, sensor 36 in thisexample. As described above, both sensors are coolant temperaturesensors, but sensor 34 in addition acts as an overheat sensor. It isdesirable to have the system adapt so that the flow of coolant throughthe heater can travel from either fitting 40 to fitting 42 or fromfitting 42 to fitting 40. The choice depends upon the plumbingrequirements of a particular installation, for example. In such a systemwith two sensors is also desirable to have the system operate even ifone sensor is removed or if one sensor becomes dysfunctional.

[0099]FIG. 9 is a chart which sets out various temperature designationsand ranges of temperatures employed by the invention. The heaternormally cycles within the Normal range of temperatures between theCurrent Cycle On Threshold and the Cycle Off Threshold. The Cycle OffThreshold is a fixed first value, 85° C. for the example of FIG. 11. Onthe other hand, the Current Cycle On Threshold, or cycle-on temperature,varies between a second value, the Minimum Cycle On Threshold, 65° C. inthe case of FIG. 11, to the Maximum Cycle On Threshold, namely 78° C. inthe case of FIG. 11. There is also a third value, an Overheat Thresholdwhich is, for example 90° C. in FIG. 10, which, when sensed, results inshutting the heater down. In addition there is the Open Threshold. Whenthe control module receives a voltage reading from a sensor equivalentto a temperature lower than the Open Threshold, then this indicates anopen circuit and that the sensor has faulted. Likewise there is a ShortThreshold which, when exceeded, indicates that the sensor has shorted.

[0100] A temperature within the range between the Current Cycle OnThreshold and the Cycle Off Threshold is considered to be within theNormal range. Temperatures below the Current Cycle On Threshold and theOpen Threshold are considered in the Low temperature range. Temperaturesbelow the Open Threshold indicate a Faulted condition.

[0101] Temperatures above the Maximum Cycle On Threshold and below theShort Threshold are considered within the Warm range. Howevertemperatures above the Cycle Off Threshold and below the Short Thresholdare considered in the High range. Temperatures above the OverheatThreshold, but below the Short Threshold are in the Overheat range.Finally temperatures above the Short Threshold show a Faulted condition.

[0102] The heater may be configured to expect one or two temperaturesensors. The temperature sensor may be connected to either sensorconnection on the control module. When the system is configured toexpect two temperature sensors, the coolant flow through the heatexchanger may be non-specific. This object is achieved by combining thevalues of the two sensors into a single overall status according to thefollowing table: TABLE 1 Temp Sensor Temperature Sensor 1 2 F OH H W N LF F F F F F F OH F OH OH OH OH OH H F OH H H H H W F OH H W W W N F OH HW N N L F OH H W N L

[0103] In the above table the temperature ranges in the upper row arethose sensed by sensor 1. The temperature ranges in the left-hand columnare those sensed by sensor 2. F indicates a temperature in the Faultedrange, OH a temperature in the Overheat range, H a temperature in theHigh range, W a temperature in the Warm range, N a temperature in theNormal range and L a temperature in the Low range.

[0104] Alternatively there may be conditions when only one sensor isrequired, but actually two are present. In this case the control moduledoes not attempt to determine which one is present. The table belowdefines the overall condition assuming that the absent sensor appearsFaulted/open. When there is only one sensor present or required, and ifit is faulted, then the control module does not know which sensor isfaulted, so faults on both sensors are generated even though there isonly one. TABLE 2 Temp Sensor Temperature Sensor 1 2 F OH H W N L F F OHH W N L OH OH OH OH OH OH OH H H OH H H H H W W OH H W W W N N OH H W NN L L OH H W N L

[0105] For prior art utilizing an NTC thermistor, it is possible thesmall amount of moisture or corrosion across the sensor leads cansimulate a cold temperature reading. This may cause the heater to firewith a false low reading, and may allow the heater to operateindefinitely if the reading does not change.

[0106] An algorithm is used to detect a temperature sensor that is notconsidered open or shorted, but stuck at some level. It is considered aDelta-T fault if at least one temperature level does not increase by aminimum Delta-T (3° C. in this example) from the time the heater entersthe Ignition state until it has been in the Run/Reignition states for aDelta-T check time (60 seconds in this example). If either temperatureincreases by the minimum Delta-T or more, then there is no Delta-Tfault. Otherwise a Delta-T fault is indicated for each sensor (which isnot faulted open/short) whose value was less than a maximum initialtemperture ( 25° C. in this example) at cycle-on time.

[0107] A further application of this algorithm is to operate it at alltimes that the burner is firing, and evaluate the temperature readingagainst typically anticipated values.

[0108] While the controls and methods described above are particularlyadapted for transit vehicle heaters and other vehicle heaters, they mayalso be useful, with some alterations, for use with other heaters orother heat transfer devices such as furnaces or air conditioners. Airconditioners typically cycle on and off between fixed higher, cycle-ontemperatures and fixed, cycle-off temperatures. The invention can beutilized for example to vary the cycle-on temperature to maintain adesirable average temperature.

[0109] Referring back to FIG. 3, the blower 60 includes a fan bladeassembly 61 which is disc-shaped and has a plurality of blades 63. Asshown in FIG. 22, there is a magnet 65 mounted on the assembly 61between a pair of projections 67 and 69. CPU board housing 90, shown inFIG. 3 and FIG. 22, houses circuit board 91 which has a Hall effectsensor 93 on the end thereof which faces the fan blade assembly. TheHall effect sensor is aligned with the magnet so that the magnet passesthe Hall effect sensor on each rotation of the fan blade assembly and,accordingly, on each rotation of the motor 64. The Hall effect sensortherefore acts as a speed sensor which is responsive to rotationalspeeds of the fan.

[0110] A programmable control module mounted on the circuit board 91 isoperatively connected to the Hall effect sensor and includes a closedloop feedback control for the motor as shown in FIG. 23. A desired motorspeed is inputted at 250 and the processor compares this value at 252 tothe momentarily measured value 254 as sensed by the sensor. Thecalculated error 256 is inputted into control module 258 which drivesprocess 260 with the value 262 to change its output 264.

[0111] The use of speed control provides significant advantages overearlier vehicle heaters where speed control has not been used.Accordingly, motor speed varied as much as 50 percent depending upon thevoltage supplied to the heater. The addition of speed control means thatthe speed of the motor is independent of voltage and the output of theheater can be regulated by selecting a particular motor speed which willgive the heater the required amount of fuel and air for the designatedoutput. Furthermore, the heater can be a single speed heater or avariable speed heater which accordingly can change the output. Forexample, the output could be increased initially to heat up a vehicleand then decreased to maintain a steady temperature.

[0112] The heater 30 has a backup speed control system in case offailure of the system described above including, for example, failure ofthe Hall effect sensor or physical dislocation of the magnet. Thecontrol module includes a lookup table. It looks up the voltage suppliedto the heater in the lookup table and uses pulse width modulation toyield the desired rotational speed for the motor. For example, Table 3below shows that for desired rotational speed of 3600 rpm, the requiredPWM at a supply voltage of 12 volts is 85 percent. TABLE 3 Volts PWM 9100 10 95 11 90 12 85 13 83 14 80 15 78 16 76 17 74 18 72 18 70 19 68 2066 21 64 22 62 23 61 24 60 25 59 26 58 27 57 28 56 29 55 30 54

[0113] Pulse width modulation is used to reduce the speed to therequired amount even if the voltage is higher. During operation of theHall effect sensor, the table is constantly updated to indicate theamount of pulse width modulation required to yield the correctrotational speed for a particular voltage applied to the heater. If theHall effect sensor fails, then speed control is maintained utilizingthis table. Effectively the control module strips off voltages above 9volts in the above example.

[0114] The use of the speed control system utilizing pulse widthmodulation allows the heater to be used for electrical systems havingdifferent voltages. In this example the heater 30 runs at 9 V, but canbe utilized on 12 V or 24 V systems. The speed controller strips off thevoltages above 9 V as mentioned above. Also the output of the heater canbe increased or decreased by increasing or decreasing rotational speedof the motor. A few other modifications are necessary including changingthe nozzle 134. Different motors are not required for different heateroutputs, but rather a single motor can be used for different heatingcapacities unlike the prior art. This reduces the number of componentswhich must be ordered and stored. A personal computer utilizingappropriate software can be connected to a port on the heater and usedto change the speed of the motor has desired.

[0115] Before the control module commences the combustion process, itexercises selective heater components to allow a service technician todirectly observe and verify operation of these loads. This facilitatestroubleshooting and eliminates the requirement for special test tools.

[0116] In the heater 30, the status of the flame sensor 149, shown inFIG. 20, is mirrored by an indicator light 161 shown in FIG. 2. Thisremoves the need for a sight glass to allow an operator or technician toview the combustion area for the presence or absence of the flame. Asdescribed above, the flame sensor is integrated into the control module.The operation of the flame sensor should be independently verified sothat the entire control module is not replaced for what might be acombustion related problem. With the heater switched off, but with powersupplied to the heater, the burner head is removed from the heaterassembly and a flashlight is directed onto the flame sensor. If theindicator light turns on, then the flame sensor functionality isconfirmed. This can eliminate the flame sensor as a potential problemwhen troubleshooting.

[0117]FIG. 31 shows a coolant pump 151 which is connected to heater 30by coolant conduit 153 and to the cooling system of engine 155. A cable157 connects the pump to the heater and supplies the pump with powerwhen operation of the pump is required.

[0118] The coolant pump may have a current limit which is less than theinrush current encountered when the coolant pump motor is started. Forexample, the current limit for the motor may be 10 amps, but the inrushcurrent may be 20 amps. A soft start may be employed so as to reduce thecurrent supplied to the motor when the motor starts. In the case where alarge pump is utilized, it must be indirectly driven through the use ofa relay. However, soft starts may cause chatter in the relay. Thiscauses the relay to eventually fail. Accordingly, the software for thecontrol module uses a special procedure to turn on the coolant pumpoutput. This is shown in detail in the flowchart of FIGS. 28-29.

[0119] The software initially turns on the coolant pump output. If theload current exceeds a preset maximum, the hardware turns off the outputand asserts its shut-off line. One millisecond after turning on theoutput, the software checks the shut-off line. If the coolant pump isstill shutting off after two seconds, then the Control module declares acoolant pump fault. At any of the shut-off checks on one millisecondintervals, if the shut-off line is not asserted, then the Control modulesets up a one second timer. If there are no further shut-offs by thetime that the timer expires, then the Control module declares the pumpsuccessfully started and any subsequent shut-offs are declared coolantpump faults. However if a shut-off does occur before the one secondtimer expires, then the Control module resumes its one millisecond checksequence (it is still within two seconds of the start of the softwareprocedure). This procedure essentially results in a 1 kHz variable dutycycle pulse width modulation (PWM) that lasts no longer than twoseconds, with successful starts known to have been running for at leastone second without faltering.

[0120] Using this approach large loads with an inrush current exceedingthe preset maximum will be soft-started, thus protecting the controlmodule from low-voltage transients, and protecting the load fromdemagnetization (only if it is a motor). Loads with inrush currentsbelow the preset maximum will be hard-started. When using a relay todrive a large coolant pump, this prevents relay chatter and prolongsrelay life.

[0121] Essentially this means that a soft start is selectively used ifthe current is above a certain level and hard start is used if thecurrent is below this level to extend relay life. The soft start turnson and off rapidly like a pulse width modulation.

[0122] During starting of the vehicle engine, the voltage supply to theheater drops as the engine is cranked by the starter motor. The voltagethen jumps when the alternator becomes operational. This voltage jumpmay show a false high current fault and consequently problems for theoperator. The invention addresses this problem by looking for rapidvoltage changes when an overcurrent condition occurs.

[0123] The shown in FIG. 29 when the vehicle is started and the Controlmodule is operating, the Control module sees a drop in supply voltageduring engine cranking, perhaps for several seconds. The motor speedcontrol will probably increase the motor duty cycle to compensate. Thisdrop in supply voltage is followed by a sudden increase in supplyvoltage when the alternator becomes operational. Such a rapid increasein supply voltage could result in motor and/or coolant pump overcurrentfaults.

[0124] The means of overcoming this problem is shown in the flowchart ofFIGS. 24-27. The Control module continuously keeps track of whetherthere has been a large change in supply voltage. Supply voltage levelwas measured 10 times per second with the last 8 samples retained. Aseach new sample is obtained, the Control module compares it with thesample taken 0.7 seconds ago. If the voltage rose by more than 1 volt, a1 bit is shifted into a 32-bit shift register, allowing up to 3.2seconds of history. Otherwise, once per 0.1 seconds, a zero is shiftedin. If the voltage fell by more than 1 volt, a 1 bit is shifted into aseparate 32-bit shift register, allowing up to 32 seconds of history.Once per second a zero is shifted in.

[0125] When a coolant pump or motor (peak or average) overcurrent faultoccurs, the Control module checks to see if any rise events occurred inthe last 2 seconds, or any fall events occurred in the last 30 seconds.If so, then the apparent coolant pump or motor current fault is declareda dV fault and essentially ignored. The fault is logged using a new codeindicating rising and/or falling supply voltage. The Finite StateMachine logic which runs the control module proceeds to a Purge Errorstate. The Error Count does not increment and the indicator light doesnot blink. It will be readily understood that the values given above areby way of example and would be altered in different embodiments.

[0126] With reference to the flowchart of FIG. 32, the inventionincludes provision for taking care of a sputtering flame caused, forexample, by air bubbles in the fuel. These air bubbles can cause theflame to sputter or go out. If this occurs, then it is necessary torestart combustion. The invention utilizes two timers, a 15 secondflame-on timer and a 10 second flame-out timer. While operating, if theflame extinguishers, then the flame sensor indicates that there is noflame and this turns on the ignition. This attempts to reignite theflame. While the heater is running normally (i.e. with a flame) aflame-on timer runs as long as the heater is in the Run state. Thistimer is frozen in the Reignition state. The Flame-on timer and theFlame-out timer are reset when the Flame-on timer times out after beingin Run state for 15 seconds.

[0127] A flame-out timer keeps track of how long the flame has been out.After being out for 10 seconds, a flame-out fault is declared. If theflame reignites, the unit returns to the Run state. The Flame-out timeris not cleared when the unit returns to the Run state rather theFlame-out timer is frozen in case the flame goes out again right away,and the system returns to the Reignition state again.

[0128] The 10 seconds for the Flame-out timer and 15 seconds for theFlame-on timer are significant. The system tolerates 10 seconds/25seconds with the flame out. In other words, the flame may be out 40percent of the time and the heater continues to run. Any more, then theheater will stop since this usually indicates a fault such as a leakingfitting.

[0129] The heater described above and shown in the drawings is anauxiliary heater for buses and trucks. Engine coolant is pumped throughthe heat exchanger which surrounds the combustion chamber. The heaterburns vehicle fuel. There are two manually operated switch inputs: amain toggle/rocker switch; and a pre-heat momentary push-button switch.The unit also has two inputs that come from the engine or an electronicengine controller. There is a coolant pump input which allows the enginecontroller to turn the unit's coolant pump on when the unit is otherwiseoff. There is also a supplemental input which directs the Control moduleto produce supplemental heat for the passenger compartment.

[0130] The unit has control over four primary devices. The first is theblower motor which blows air through the combustion chamber and providesuction for the fuel. The air movement provides oxygen for combustion,removes exhaust gases and cools the chamber after the flame is put out.The second is the coolant pump that helps move liquid engine coolantbetween the input and output ports of the heat exchanger. The third is asolenoid that controls a fuel valve. The fourth is the spark ignitorused to start the fuel burning. The ignitor is turned off after the fuelstarts to burn. Normally the flame continues until the supply of fuel isswitched off.

[0131] The unit has additional inputs to sense the presence of a flame,measure coolant temperature, and detect over/under voltage and otherfaults, and has additional outputs for an indicator lamp and to powerauxiliary/accessory devices. Non-volatile memory is used to record hoursmeters and keep an event/fault log. The unit has a serial diagnosticport which allows a remote PC to access/control unit operation.

[0132] There is a heating cycle which is defined as a sequence ofautomatic operations by the Control module beginning with detectingtemperature below the cycle on threshold and starting combustion, andending with detecting temperature above the cycle off threshold andextinguish in combustion.

[0133] Once a heating cycle starts, there may be fuel and/or hot exhaustgases in the combustion chamber. When the heating cycle ends, whether ornot it terminates successfully, the Control module continues to run theblower motor for a period of time in order to clear out in cool down thecombustion chamber. This process is known as purging.

[0134] The Control module of the preferred embodiment has an RS232communication port over which it can interact with a diagnostic programrunning on the standard PC.

[0135] Many aspects of the Control module operation are governed byparameters accessible and modifiable via the datalink.

[0136] The behavior of the heater is specified by a finite state machinewith 16 defined states in the preferred embodiment. However the unit isconsidered to be operating in one of four modes discussed below.

[0137] The Normal Mode is the primary mode for the unit. Operationduring this mode depends on the state of the main switch. When the mainswitch is on, the coolant pump runs continuously, and the burners turnedon/off according to temperature set points (i.e. similar to a thermostatfor a house furnace). When the main switch is off, the burners staysoff, but the coolant pump runs whenever requested by the enginecontroller.

[0138] Supplemental mode is similar to Normal mode (with the Main switchon), except that the coolant pump does not run continuously. InSupplemental mode, the coolant pump only runs while the burner is on orwhen requested by the engine controller. This mode is selected byturning on the Supplemental input (while keeping the Main switch off).Supplemental mode is canceled when the Main switch is turned on.

[0139] Preheat mode is similar to the Normal mode (with the main switchon), except that it automatically shuts off after 90 minutes. Preheatmode is entered when the operator presses the Preheat pushbutton switchmomentarily. The switch is only honored when both main and supplementalinputs are off. Preheat mode is canceled when either of these otherswitches is turned on.

[0140] There are three levels of severity of failure conditions whichmay occur. The first level is noncritical. Some aspect of the unit hasfailed, but it still can perform its basic heater function and thecurrent heating cycle is allowed to continue.

[0141] The second level is critical. Here the unit cannot continue thecurrent heating cycle any longer. The cycle is terminated, but another(automatic) heating cycle is permitted regardless of how many differentcritical faults have occurred within the cycle.

[0142] If two consecutive heating cycles are terminated in this manner,it is considered catastrophic. Here the unit cannot automaticallyinitiate any more heating cycles. Operator intervention is required. Forexample, and overheat fault is considered catastrophic.

[0143] Once a fault has been recognized, and acted upon, the controlmodule must consider the fault condition to be cleared before acting onit again. This prevents a single event from triggering repeated logentries. The control module remembers which faults are currently activeand resets this memory under the following conditions:

[0144] For critical and catastrophic faults, all such faults are resetupon a transition from a class B state to a class A state. Purge, purgeerror, purge shutdown, purge off, shutdown and shutdown override, alldiscussed below, are class B states. All others are class A states.

[0145] For noncritical faults, all such faults are reset as above forcritical faults and also on entry to the off state and on exit from thepurge state. Again these states are discussed below.

[0146] The operation of the heater will now be explained with referenceto the various operational states thereof. The operation of the heateris specified by a finite state machine (FSM) with the following states.In general all of the states monitor switch inputs for mode changes,exit Preheat mode when time expires and check for faults on givenoutputs and inputs of interest.

[0147] Powered Off—This represents the state of the electronic controlmodule when it is powered off. When the power is turned on, the heaternormally enters the (heater) Off state.

[0148] Off—The heater is off in this state. The electronic controlmodule however only stops running when the power supply to the controlmodule is disconnected. All operator switches are off and the unit isconsidered to be in Normal mode awaiting operator or engine controlmodule input.

[0149] The unit is intended to be powered, normally by the vehiclebattery, at all times. Therefore the heater has a low-power sleep modewhile in this state. Any manual switch operation, request from theengine controller or diagnostic port connection will wake it up.

[0150] While in the Off state, the indicator light is used to show thepresence or absence of the flame as detected by the flame sensor. Thisis to permit a service technician to verify the functionality of theflame sensor.

[0151] Ignition Check—This state occurs just after the heater has beenswitched on while in the Off state using the Main switch. The unit turnsthe ignitor on for five seconds (Ignition Check timeout parameter),allowing the service technician to verify ignitor functionality. Ignitorfaults are not checked during this period. The state will terminateprematurely, and the unit returns to the Off state if the Main switch isturned off, otherwise the next state is Standby.

[0152] Standby—The unit in this state has been switched on by one of theoperator switches, but the burner is not on. The unit monitors coolanttemperature and initiates the process to turn on the burner when thetemperature drops below a lower threshold. The coolant pump is runningcontinuously in this state. The state may occur in any of the threeoperating modes. However the only way it can occur in Supplemental modeis if the engine controller requests that the coolant pump run.

[0153] Standby Supp—This state is only for Supplemental mode. It issimilar to the Standby state except that the coolant pump is off. Theengine controller does not request the coolant pump to run. If theengine controller does request the coolant pump, then the unit changesto Standby stage. If the burner needs to be turned on, the unit goes tothe Prerun state.

[0154] Prerun—This state occurs only for the Supplemental mode. Thepurpose of this state is to run the coolant pump for thirty seconds. Itthen checks if the temperature sensed still requires the burner to beturned on. This is because the coolant pump has been off and the unitmay not be reading the true coolant temperature. The heat from theengine itself may be sufficient and there may be no need to turn on theburner.

[0155] Precheck—This is the first of a sequence of states the unit goesthrough to turn on the burner. Power is applied to the ignition module,but sparking is not enabled. The state lasts about 0.5 seconds, givingthe unit time to check for a few types of fault conditions. The checksperformed include:

[0156] is the flame already on? (May indicate a faulty flame sensor)

[0157] are the temperature sensors okay?

[0158] is there an overheat condition (combustion chamber too hot)?

[0159] is system voltage within acceptable operating range?

[0160] are there ignition module, fuel solenoid or coolant pump faultspresent?

[0161] Preignition—This is the second of a sequence of states turn onthe burner. The blower motor is turned on and ignition module sparkingis enabled at this point. The fuel valve is kept closed. The state lastsfor about five seconds, giving the unit time to verify motor startup anddetect ignition module faults.

[0162] Ignition—This is a third of a sequence of states that the unitgoes through to turn on the burner. The fuel valve is opened at thispoint. The objective is to ignite the fuel. The state lasts about thirtyseconds. During this interval, in addition to the usual array of faultconditions, the unit monitors whether the flame is out. At the end ofthis interval, if the flame had not been on sufficiently long enough(see Start Criteria parameter), then the sequence is aborted because theburner failed to start.

[0163] Run—This is the final state in the sequence the unit goes throughto turn on the burner. Ignition module sparking is turned off at thispoint. Fuel should continue to burn. The unit remains in the state untilcoolant temperature reaches the upper threshold, the Main orSupplemental switch is turned off, or some critical fault is detected.Should the flame go out, the unit attempts reignition by going to theReignition state. When it is time to terminate the current heatingcycle, the unit goes into one of the Purge states to clear thecombustion chamber of exhaust gases and cool it down.

[0164] Reignition—When the flame goes out during the Run state, the unitattempts to reignite it in this state. Ignition module sparking isre-enabled. The state lasts for up to ten seconds or until a flame issensed again. A flame-out timeout timer keeps track of how long theflame has been out. After being out for ten seconds, a flame-out faultis registered. If the flame reignites, the unit returns to the Run statewith sparking off. The Reignition flame-out timeout timer is not clearedwhen the unit returns to the Run state. Rather the Reignition flame-outtimeout timer is frozen in case the flame goes out again right away, andthe heater returns to the Reignition state again. The second timer knownas the Reignition flame timeout timer runs only in Run state (and isfrozen while in Reignition state). The reignition flame timeout timer isreset when the Reignition flame timeout timer times out (after being inthe Run state 15 seconds), the Reignition flame timeout timer alsorestarts.

[0165] Purge/Purge Off/Purge Error/Purge Shut down—After the burner isturned off at the end of the heating cycle, the combustion chamber iscleared of exhaust gases and cooled by running the blower motor forabout 2 minutes. There are four variations of the Purge state, dependingon how the cycle ended and what the state of the unit will be after thepurging is completed.

[0166] Purge—normal termination of heating cycle because uppertemperature threshold was reached. Unit remains in current operatingmode with next state being Standby.

[0167] Purge Off—normal termination of heating cycle because operatorswitched off the unit or Pre-heat interval timed out. After purge periodexpires, unit goes to Off state in Normal mode.

[0168] Purge Error—heating cycle terminated due to a critical failure.While purging, an error code is displayed on the indicator, but afterthe purge period expires. The unit remains in its current operation modewith the next state being Standby.

[0169] Purge Shutdown—heating cycle terminated due to a catastrophicfailure. An error code is displayed on the indicator, and after thepurge period expires, the next state is Shutdown (the error codecontinues to be displayed).

[0170] When the blower motor is on during a purge state, it is importantthat the blower be kept running if possible to adequately cool theburner and vent exhaust gases and unburnt fuel. About one second after ablower motor fault, the motor output is retried. Blower motor PWMgradually ramps up to the target motor speed. This may take severalseconds. There is one exception to this motor retry while in purgestrategy, namely if the flame sensor detects a flame (see Purge flametimeout parameter), then the motor is turned off (and not retried) in anattempt to extinguish the flame.

[0171] Shutdown—The unit in the state has automatically turned itselfoff due to a catastrophic failure. The unit remains in this (or C.P.Override) state until operator presence is indicated by switchoperation. If Main and/or Supplemental switches were on at the time offailure, the operator must switch them both off. If the Preheat mode wasactive at the time of failure (Main and Supplemental switches must havebeen off), the operator must turn the Main or Supplemental switch on(This does not engage the heater in the corresponding mode, rather theunit stays in Shutdown state, but no longer considers itself in Reheatmode.) and off again. The unit then returns to Off state in Normal mode.

[0172] C.P. Override (Shutdown Override)—While the unit is in Shutdownstate, the engine controller can still request that the coolant pumprun. The state is essentially identical to Shutdown except the coolantpump is turned on. When the engine controller removes its request, theunit returns to Shutdown state. If there is a coolant pump failure, itis retried every 10 seconds.

[0173] C.P. Run (Off Override)—While the unit is in Off state, theengine controller can still request that the coolant pump run. The stateis essentially identical to Off state, except the coolant pump is turnedon. When the engine controller removes its request, the unit returns toOff state. If there is a coolant pump failure, it is retried every 10seconds.

[0174] It will be understood by someone skilled in the art that many ofthe details provided above are by way of example only and can be alteredor deleted without departing from the scope of the invention which is tobe interpreted with reference to the following claims.

What is claimed is:
 1. A method of maintaining a temperature between afirst temperature value and a second, variable temperature value, themethod utilizing a heat transfer device capable of cycling through aseries of cycles, the device being capable of changing the temperaturefrom the second value to the first value when operational, during eachsaid cycle the device transferring heat for a first period of time whenthe temperature reaches the second value until the first value isreached and thereafter the device ceasing to transfer heat, said firstperiod of time being compared to a desired cycle-on period of timeduring each said cycle and said second value then being adjusted for asubsequent cycle of the device so said first period of time when thedevice transfers heat approaches said desired cycle-on period of time.2. The method as claimed in claim 1, wherein the device is a heater andthe first value is higher than the second value.
 3. The method asclaimed in claim 2, wherein the heater has a burner, the burner becomingoperational when the second temperature value is reached and becomesnonoperational when the first temperature value is reached.
 4. Themethod as claimed in claim 3, the device being a fluid heater and thefirst value, and the second value being temperatures of a fluid heatedby the heater.
 5. The method as claimed in claim 4, wherein the fluid isan aqueous solution including water and anti-freeze.
 6. The method asclaimed in claim 5, wherein the heater is a transit vehicle heater. 7.The method as claimed in claim 3, wherein the second temperature valueis increased after each cycle where the first period of time exceeds thedesired cycle-on period of time.
 8. The method as claimed in claim 7,wherein the second temperature value is decreased after each cycle wherethe desired cycle-on period of time exceeds the first period of time. 9.The method as claimed in claim 3, wherein the heater becomes operationalfor another cycle after the heater is nonoperational for a preselectedperiod of time, whether or not the second temperature is reached.
 10. Anapparatus for maintaining a temperature between a first temperaturevalue and a second temperature value, the apparatus including a heattransfer device capable of cycling through a series of cycles, thedevice being capable of changing the temperature from the second valueto the first value when operational, and controls which control thedevice so during each said cycle the device transferring heat for afirst period of time when the temperature reaches the second temperaturevalue until the first value is reached and thereafter the device ceasestransferring heat, said controls comparing the first period of time to adesired cycle-on period of time during each said cycle and said controlsadjusting the second temperature value for a subsequent cycle of thedevice so said first period of time when the device transfers heatapproaches said desired cycle-on period of time.
 11. The apparatus asclaimed in claim 10, wherein the device is a heater and the first valueis higher than the second value.
 12. The apparatus as claimed in claim11, wherein the heater has a burner, the burner becoming operationalwhen the second temperature value is reached and becomes nonoperationalwhen the first temperature value is reached.
 13. The apparatus asclaimed in claim 13, the device being a fluid heater and the first valueand the second value being temperatures of a fluid heated by the heater.14. The apparatus as claimed in claim 14, wherein the fluid is anaqueous solution including water and anti-freeze.
 15. The apparatus asclaimed in claim 13, wherein the heater is a transit vehicle heater. 16.The apparatus as claimed in claim 13, wherein the controls increase thesecond temperature value after each cycle where the first period of timeexceeds the desired cycle-on period of time.
 17. The apparatus asclaimed in claim 17, wherein the controls decrease the secondtemperature value after each cycle where the desired cycle-on period oftime exceeds the first period of time.
 18. The apparatus as claimed inclaim 13, wherein the controls cause the heater to become operationalfor another cycle after the heater is nonoperational for a preselectedperiod of time, whether or not the second temperature value is reached.19. A heater for a liquid including controls for maintaining atemperature of the liquid between a first value and a second lowervalue, the heater being capable of cycling through a series of cycles,the controls controlling the heater so during each said cycle the heaterheats for a first period of time when the liquid cools to the secondtemperature value until the liquid is heated to the first value andthereafter the heater stops heating, said controls comparing the firstperiod of time to a desired cycle-on period of time during each saidcycle and said controls adjusting the second temperature value for asubsequent cycle of the device so said first period of time when theheater heats approaches said desired cycle-on period of time.
 20. Theheater as claimed in claim 19, wherein the heater has a burner, theburner becoming operational when the second temperature value is reachedand becomes nonoperational when the first temperature value is reached.21. The heater as claimed in claim 19, wherein the liquid is an aqueoussolution including water and anti-freeze.
 22. The heater as claimed inclaim 19, wherein the heater is a transit vehicle heater.
 23. The heateras claimed in claim 19, wherein the controls increase the secondtemperature value after each cycle where the first period of timeexceeds the desired cycle-on period of time.
 24. The heater as claimedin claim 19, wherein the controls decrease the second temperature valueafter each cycle where the desired cycle-on period of time exceeds thefirst period of time.
 25. The heater as claimed in claim 19, wherein thecontrols cause the heater to commence heating for another cycle afterthe heater has stopped heating for a preselected period of time, whetheror not the second temperature value is reached.
 26. The heater asclaimed in claim 19, wherein the controls include a temperature sensor.27. The heater as claimed in claim 19, wherein the controls include aprogrammable control module.
 28. A control system for a heat transferdevice, the control system maintaining a temperature between a firstvalue and a variable second value, the heat transfer device beingcapable of cycling through a series of cycles, the device being capableof changing the temperature from the second value to the first value,the control system controlling the device so during each said cycle thedevice transfers heat for a first period of time when the temperaturereaches the second temperature value until the first value is reachedand thereafter the device stops transferring heat, said controlscomparing the first period of time to a desired cycle-on period of timeduring each said cycle and said control system adjusting the secondtemperature value for a subsequent cycle of the device so said firstperiod of time when the device transfers heat approaches said desiredcycle-on period of time.
 29. The control system as claimed in claim 28,wherein the device is a heater and the first value is higher than thesecond value.
 30. The control system as claimed in claim 28, wherein theheater has a burner, the burner becoming operational when the secondtemperature value is reached and becomes nonoperational when the firsttemperature value is reached.
 31. The control system as claimed in claim30, the device being a fluid heater, the first value and the secondvalue being temperatures of a fluid heated by the heater.
 32. Thecontrol system as claimed in claim 28, wherein the heater is a transitvehicle heater.
 33. The control system as claimed in claim 28, whereinthe control system increases the second temperature value after eachcycle where the first period of time exceeds the desired cycle-on periodof time.
 34. The control system s as claimed in claim 28, wherein thecontrol system decreases the second temperature value after each cyclewhere the desired cycle-on period of time exceeds the first period oftime.
 35. The control system as claimed in claim 28, wherein the controlsystem causes the heater to heat for another cycle after the heaterstops heating for a preselected period of time, whether or not thesecond temperature value is reached.
 36. The control system as claimedin claim 28, including a temperature sensor and a programmable controlmodule.
 37. Temperature controls for a liquid heater having a jacketcontaining the liquid, the jacket having an inner heated wall and anouter wall spaced apart from the inner wall, the controls including atemperature sensor capable of fitting to the jacket so that the sensorcontacts the inner wall and the liquid, and a control module whichmonitors temperature sensed by the temperature sensor, the controlmodule causing the heater to stop heating when the temperature is heatedby the heater to a first temperature value and causing the heater toheat when the liquid cools to a temperature second value, the controlmodule sensing overheating when a temperature sensed by the sensorincreases to a third temperature value, higher than the first value,when the heater is not heating, and then causing the heater to shutdown.
 38. A heater for a liquid, the heater having a jacket containingthe liquid, the jacket having an inner heated wall and an outer wallspaced apart from the inner wall, and temperature controls operativelyconnected thereto, the controls including a temperature sensor capableof fitting to thejacket so that the sensor contacts the inner wall andthe liquid, and a control module which monitors temperature sensed bythe temperature sensor, the control module causing the heater to stopheating when the temperature is heated by the heater to a firsttemperature value and causing the heater to heat when the liquid coolsto a second temperature value, the control module sensing overheatingwhen a temperature sensed by the sensor increases to a third temperaturevalue, higher than the first temperature value, when the heater is notheating, and then causing the heater to shut down.
 39. The liquid heateras claimed in claim 38, wherein the sensor has a resiliently biased tipwhich contacts the inner wall.
 40. A method of controlling temperaturefor a liquid heater having a jacket containing the liquid, the jackethaving an inner heated wall and an outer wall spaced apart from theinner wall, the method including fitting a temperature sensor to thejacket so that the sensor contacts the inner wall and the liquid,monitoring temperature sensed by the temperature sensor, causing theheater to stop heating when the temperature is heated by the heater to afirst temperature value and causing the heater to heat when the liquidcools to a second temperature value lower than the first value,detecting when temperature sensed by the sensor increases to a thirdvalue, higher than the first value, when the heater is not heating,treating said third value as an overheat condition and then causing theheater to shut down.
 41. A temperature control apparatus for a liquidheater having two fittings for connecting liquid conduits to the heater,one of the fittings being either an inlet or an outlet for liquid,another of the fittings being another of the inlet or the outlet for theliquid, the heater having a first temperature sensor adjacent to saidone fitting and a second temperature sensor adjacent to said anotherfitting, the apparatus including a memory having values for a firsttemperature value, where the heater stops heating, a second temperaturevalue where the heater commences heating, and a third temperature value,higher than the first value, where the heater is shut off, both sensorsbeing capable of sensing the first temperature value and the secondtemperature value and one of the sensors also sensing the thirdtemperature value, and controls which compare temperature readings fromeach of the sensors with each of the first temperature value, the secondtemperature value and the third temperature value, the heater having anoverheat condition when a temperature exceeds the third temperaturevalue, a high condition when a temperature exceeds the first temperaturevalue, a normal condition when a temperature is between the secondtemperature value and the first temperature value and a low conditionwhen a temperature is less than the second temperature value, theapparatus indicating the overheat condition when one of the sensorsindicates a temperature exceeding the third temperature value, the highcondition when either sensor indicates a temperature exceeding the firsttemperature value, but neither sensor indicates a temperature exceedingthe third temperature value, the normal condition when either sensorindicates a temperature exceeding the second temperature value, butneither sensor indicates a temperature exceeding the third temperaturevalue, and a low condition when both sensors indicate temperatures lessthan the second temperature value.
 42. The apparatus as claimed in claim41, wherein the second temperature value is variable between a minimumcycle-on temperature and a maximum cycle-on temperature, the heaterhaving a warm condition when a temperature exceeds the maximum cycle-ontemperature, the apparatus indicating the warm condition when one of thesensors indicates a temperature exceeding the maximum cycle-ontemperature, but neither sensor indicates a temperature exceeding thefirst temperature value.
 43. The apparatus as claimed in claim 41,wherein the heater has a faulted condition when a temperature readingexceeds a short threshold which indicates a short circuit, the shortthreshold exceeding the third temperature value, or when a temperaturereading is less than an open threshold which indicates an open circuit,the open threshold being less than the minimum cycle-on temperature, theapparatus indicating the faulted condition when either sensor exceedsthe short threshold or is less than the open threshold, the overheatcondition being indicated when either of the sensors exceeds the thirdtemperature value, but does not indicate the faulted condition.
 44. Amethod of controlling temperatures in a liquid heater having twofittings for connecting liquid conduits to the heater, one of thefittings being either an inlet or an outlet for liquid, another of thefittings being another of the inlet or the outlet for the liquid, theheater having a first temperature sensor adjacent to said one fittingand a second temperature sensor adjacent to said another fitting, themethod including retaining values for a first temperature, where theheater stops heating, a second temperature where the heater commencesheating, and a third temperature where the heater is shut off, bothsensors being capable of sensing the first temperature value and thesecond temperature value and one of the sensors also sensing the thirdtemperature value, comparing temperature readings from each of thesensors with each of the second temperature value, the first temperaturevalue and the third temperature value, the heater having an overheatcondition when a temperature exceeds the third temperature value, a highcondition when a temperature exceeds the first temperature value, anormal condition when a temperature is between the second temperaturevalue and the first temperature value and a low condition when atemperature is less than the second temperature value, the overheatcondition being indicated when one of the sensors indicates atemperature exceeding the third temperature value, the high conditionwhen either sensor indicates a temperature exceeding the firsttemperature value, but neither sensor indicates a temperature exceedingthe overheat temperature, the normal condition when either sensorindicates a temperature exceeding the second temperature value, butneither sensor indicates a temperature exceeding the first temperaturevalue, and a low condition when both sensors indicate temperatures lessthan the second temperature value.
 45. The method as claimed in claim44, wherein the second temperature value is varied between a minimumcycle-on temperature and a maximum cycle-on temperature, the heaterhaving a warm condition when a temperature exceeds the maximum cycle-ontemperature, the warm condition being indicated when one of the sensorsindicates a temperature exceeding the maximum cycle-on temperature, butneither sensor indicates a temperature exceeding the first temperaturevalue.
 46. The method as claimed in claim 41, wherein the heater has afaulted condition when a temperature reading exceeds a short thresholdwhich indicates a short circuit, the short threshold exceeding the thirdtemperature value, or when a temperature reading is less than an openthreshold which indicates an open circuit, the open threshold being lessthan the minimum cycle-on temperature, the faulted condition beingindicated when either sensor exceeds the short threshold or is less thanthe open threshold, the overheat condition being indicated when eitherof the sensors exceeds the overheat temperature, but does not indicatethe faulted condition.
 47. A temperature control apparatus for a liquidheater having a first temperature sensor and a second temperaturesensor, the apparatus including a memory having values for a firsttemperature, where the heater stops heating, a second temperature valuewhere the heater commences heating, and a third temperature value wherethe heater is shut off, and controls which compare temperature readingsfrom each of the sensors with each of the first temperature value, thesecond temperature value and the third temperature value, both sensorsbeing capable of sensing the first temperature value and the secondtemperature value and one of the sensors also sensing the thirdtemperature value, the heater having an overheat condition when atemperature exceeds the third temperature value, a high condition when atemperature exceeds the first temperature value, a normal condition whena temperature is between the first temperature value and the secondtemperature value and a low condition when a temperature is less thanthe second temperature value, the apparatus indicating the overheatcondition when one of the sensors indicates a temperature exceeding thethird temperature value, the high condition when either sensor indicatesa temperature exceeding the first temperature value, but neither sensorindicates a temperature exceeding the third temperature value, thenormal condition when either sensor indicates a temperature exceedingthe second temperature value, but neither sensor indicates a temperatureexceeding the first temperature value, and a low condition when bothsensors indicate temperatures less than the second temperature value.48. The apparatus as claimed in claim 47, wherein the second temperaturevalue is variable between a minimum cycle-on temperature and a maximumcycle-on temperature, the heater having a warm condition when atemperature exceeds the maximum cycle-on temperature, the apparatusindicating the warm condition when one of the sensors indicates atemperature exceeding the maximum cycle-on temperature, but neithersensor indicates a temperature exceeding the first temperature value.49. The apparatus as claimed in claim 47, wherein the heater has afaulted condition when a temperature reading exceeds a short thresholdwhich indicates a short circuit, the short threshold exceeding the thirdtemperature value, or when a temperature reading is less than an openthreshold which indicates an open circuit, the open threshold being lessthan the minimum cycle-on temperature, the apparatus indicating thefaulted condition when both of the sensors indicate a faulted condition,whereby the heater is operational when only one sensor is present or ifone of the sensors is faulted.
 50. A method of controlling temperaturesin a liquid heater, the heater having a first temperature sensor and asecond temperature sensor, the method including retaining values for afirst temperature value, where the heater stops heating, a secondtemperature value where the heater commences heating, and a thirdtemperature value where the heater is shut off, both sensors beingcapable of sensing the first temperature value and the secondtemperature value and one of the sensors also sensing the thirdtemperature value, comparing temperature readings from each of thesensors with each of the first temperature value, the second temperaturevalue and the third temperature value, the heater having an overheatcondition when a temperature exceeds the third temperature value, a highcondition when a temperature exceeds the first temperature value, anormal condition when a temperature is between the first temperaturevalue and the second temperature value and a low condition when atemperature is less than the second temperature value, the overheatcondition being indicated when one of the sensors indicates atemperature exceeding the third temperature value, the high conditionwhen either sensor indicates a temperature exceeding the firsttemperature value, but neither sensor indicates a temperature exceedingthe third temperature value, the normal condition when either sensorindicates a temperature exceeding the second temperature value, butneither sensor indicates a temperature exceeding the first temperaturevalue, and a low condition when both sensors indicate temperatures lessthan the second temperature value.
 51. The method as claimed in claim50, wherein the second temperature value is varied between a minimumcycle-on temperature and a maximum cycle-on temperature, the heaterhaving a warm condition when a temperature exceeds the maximum cycle-ontemperature, the warm condition being indicated when one of the sensorsindicates a temperature exceeding the maximum cycle-on temperature, butneither sensor indicates a temperature exceeding the first temperaturevalue.
 52. The method as claimed in claim 50, wherein the heater has afaulted condition when a temperature reading exceeds a short thresholdwhich indicates a short circuit, the short threshold exceeding the thirdtemperature value, or when a temperature reading is less than an openthreshold which indicates an open circuit, the open threshold being lessthan the minimum cycle-on temperature, the faulted condition beingindicated when either sensor exceeds the short threshold or is less thanthe open threshold, the faulted condition being indicated when both ofthe sensors indicate a faulted condition, whereby the heater isoperational when only one sensor is present or if one of the sensors isfaulted.
 53. A heater for liquids having two fittings for connectingliquid conduits to the heater, one of the fittings being either an inletor an outlet for liquid, another of the fittings being another of theinlet or the outlet for the liquid, the heater having a firsttemperature sensor adjacent to said one fitting and a second temperaturesensor adjacent to said another fitting, the heater including a memoryhaving values for a first temperature value, where the heater stopsheating, a second temperature value where the heater commences heating,and an overheat temperature where the heater is shut off, both sensorsbeing capable of sensing the first temperature value and the secondtemperature value and one of the sensors also sensing the thirdtemperature value, and controls which compare temperature readings fromeach of the sensors with each of the first temperature value, the secondtemperature value and the third temperature value, the heater having anoverheat condition when a temperature exceeds the third temperaturevalue, a high condition when a temperature exceeds the first temperaturevalue, a normal condition when a temperature is between the firsttemperature value and the second temperature value and a low conditionwhen a temperature is less than the second temperature value, theapparatus indicating the overheat condition when one of the sensorsindicates a temperature exceeding the third temperature value, the highcondition when either sensor indicates a temperature exceeding the firsttemperature value, but neither sensor indicates a temperature exceedingthe third temperature value, the normal condition when either sensorindicates a temperature exceeding the second temperature value, butneither sensor indicates a temperature exceeding the first temperaturevalue, and a low condition when both sensors indicates temperatures lessthan the second temperature value.
 54. The heater as claimed in claim53, wherein the second temperature value is variable between a minimumcycle-on temperature and a maximum cycle-on temperature, the heaterhaving a warm condition when a temperature exceeds the maximum cycle-ontemperature, the apparatus indicating the warm condition when one of thesensors indicates a temperature exceeding the maximum cycle-ontemperature, but neither sensor indicates a temperature exceeding thefirst temperature value.
 55. The heater as claimed in claim 53, whereinthe heater has a faulted condition when a temperature reading exceeds ashort threshold which indicates a short circuit, the short thresholdexceeding the their temperature value, or when a temperature reading isless than an open threshold which indicates an open circuit, the openthreshold being less than the minimum cycle-on temperature, theapparatus indicating the faulted condition when either sensor exceedsthe short threshold or is less than the open threshold, the overheatcondition being indicated when either of the sensors exceeds the thirdtemperature value, but does not indicate the faulted condition.
 56. Aheater for liquids having a first temperature sensor and a secondtemperature sensor, a memory having values for a first temperaturevalue, where the heater stops heating, a second temperature value wherethe heater commences heating, and a third temperature value where theheater is shut off, both sensors being capable of sensing the firstamateur value and a second temperature value and one of the sensors alsosensing the third temperature value, and controls which comparetemperature readings from each of the sensors with each of the firsttemperature value, the second temperature value and the thirdtemperature value, the heater having an overheat condition when atemperature exceeds the third temperature value, a high condition when atemperature exceeds the first temperature value, a normal condition whena temperature is between the first temperature value and the secondtemperature value and a low condition when a temperature is less thanthe second temperature value, the controls indicating the overheatcondition when one of the sensors indicates a temperature exceeding thethird temperature value, the high condition when either sensor indicatesa temperature exceeding the first temperature value, but neither sensorindicates a temperature exceeding the third temperature value, thenormal condition when either sensor indicates a temperature exceedingthe second temperature value, but neither sensor indicates a temperatureexceeding the first temperature value, and a low condition when bothsensors indicates temperatures less than the second temperature value.57. The heater as claimed in claim 56, wherein the second temperaturevalue is variable between a minimum cycle-on temperature and a maximumcycle-on temperature, the heater having a warm condition when atemperature exceeds the maximum cycle-on temperature, the controlsindicating the warm condition when one of the sensors indicates atemperature exceeding the maximum cycle-on temperature, but neithersensor indicates a temperature exceeding the second temperature value.58. The heater as claimed in claim 56, having a faulted condition when atemperature reading exceeds a short threshold which indicates a shortcircuit, the short threshold exceeding the the attempt or value, or whena temperature reading is less than an open threshold which indicates anopen circuit, the open threshold being less than the minimum cycle-ontemperature, the controls indicating the faulted condition when both ofthe sensors indicate a faulted condition, whereby the heater isoperational when only one sensor is present or if one of the sensors isfaulted.
 59. A heater for a liquid including a temperature sensor andcontrols for maintaining a temperature of the liquid between a firstvalue and a second lower value, the heater being capable of cyclingthrough a series of cycles, the controls controlling the heater soduring each said cycle the heater commences heating when the temperaturesensor indicates that the temperature has decreased to the lower value,the controls indicating a temperature sensor error if the temperatureindicated by the sensor does not increase by a specified amount within aspecified time after the heater commences heating.
 60. A method ofdetecting a temperature sensor error in a heater for a liquid includinga temperature sensor and controls for maintaining a temperature of theliquid between a first value and a second lower value, the heater beingcapable of cycling through a series of cycles, the controls controllingthe heater so during each said cycle the heater commences heating whenthe temperature sensor indicates that the temperature has decreased tothe lower value, the controls indicating a temperature sensor error ifthe temperature indicated by the sensor does not increase by a specifiedamount within a specified time after the heater commences heating.